The present invention relates to a method for determining correction values for correcting positional measurement errors in a machine having at least one translational movement axis, such as, in particular in a coordinate measuring machine. The invention further relates to a coordinate measuring machine having a memory for storing correction values which have been determined in accordance with such a method.
Coordinate measuring machines are typically used to measure the shape of a measurement object with high accuracy. For example, coordinate measuring machines are used to check the shape of workpieces for quality control. A coordinate measuring machine has a displacement mechanism which enables a probe to move inside a measurement volume. The probe is brought into a defined position relative to the measurement object, and subsequently the spatial coordinates of the measuring points are determined from the position of the probe in the measurement value.
The probe often carries a stylus designed for contacting the selected measuring point on the measurement object in order to initiate the determination of the spatial coordinates. As an alternative, however, there are also heads which sense the measurement object without making contact, for example by means of optical means. The present invention is independent of the type of probing. It relates to all coordinate measuring machines and, moreover, to any other machines, where a head is moved via a displacement mechanism along at least one translational axis of movement. Thus, the invention can, for example, also be applied in the case of machine tools, EDM machines or robots. The head is then, for example, a tool holding fixture.
For sake of simplicity, however, the invention is illustrated below for the preferred application of a coordinate measuring machine, since here the requirements placed on the measuring accuracy for the sensing of position are particularly high.
The current spatial position of the head is determined by means of what is called material measures (or material representations) in the case of known coordinate measuring machines. Said material measures are often glass bars on which a scale is applied. Position data for the position of the head are read off at the material measures by means of a suitable sensor system.
However, it is known that the accuracy in the determination of the spatial position of the head is limited. In other words, each measurement is affected by measurement errors. The measurement errors have various causes. These include the limited accuracy with which material measures can be produced. This holds true, in particular, in the case of large coordinate measuring machines in which the material measures must be assembled from a number of parts. However, even in the case of relatively small coordinate measuring machines small manufacturing tolerances in the material measures are already enough to affect the measuring accuracy of the machine. It can happen that only a few of a plurality of “equal” material measures are suitable for the desired accuracy of a coordinate measuring machine. Sometimes, defects in the material measures come to light only after they have been installed in the coordinate measuring machine, and this is particularly disadvantageous since the “poor” material measures must subsequently be exchanged. The production costs of a coordinate measuring machine therefore depend strongly on the quality and the costs of the material measures.
It is known to compile correction value tables (or error tables) and make them available in a memory of the coordinate measuring machine in order to reduce the inevitably remaining measurement errors. For this purpose, the head of the coordinate measuring machine is moved along defined path of movements. The positions of the head are recorded both with the position measuring devices of the coordinate measuring machine itself, and with second position measuring devices. It is usually laser interferometers and what is called inclination scales which serve as second position measuring devices in this sense. The position data of these second position measuring devices are adopted as “true” position data, and correction values are determined from the comparison with the first position data of the coordinate measuring machine. These correction values are taken into account in the measurements. Such a method is described, for example, in DE 1 638 032 A1.
The accuracy of this error correction depends, inter alia, on how “finely meshed” the network of the correction values is. The greater the spacings at which correction values are determined, the more likely it is for position-specific errors (so-called short-period errors) not to be detected and, consequently, not to be corrected. Consequently, a “finely meshed” network of correction values is desirable for a high accuracy.
However, the determination of many correction values is time consuming, particularly for large coordinate measuring machines, such as are used, for example, in order to measure entire motor vehicle bodies. Moreover, large correction value tables require a great deal of memory space in the control and evaluation unit of the coordinate measuring machine. Both factors contribute to high production costs.
On the other hand, it is just large coordinate measuring machines having assembled material measures which are particularly susceptible to position-specific errors, in particular at the joints of the multipartite scales. There is a range of proposals for avoiding or at least reducing such errors.
An overview of the prior art is given by DE 197 24 732 A1, which describes a modular scale in which the joints of the assembled scale are positioned such that they lie as distant as possible from the joints of an associated carrier element. Thus, an attempt is made here to improve the quality of assembled scales.
DE 101 62 849 A1 proposes that a scale which is shorter than the measuring volume is moved relative to the position of a number of sensors spaced apart from one another, one of the sensors respectively being selected for the measuring. The disadvantage of this proposal is a need for a plurality of sensors, which is expensive. DE 196 21 015 C2 also proposes to increase the accuracy in the region of joints of an assembled scale by making use of a number of sensors. Moreover, it is known to identify the joints of scales by specific code fields (DE 38 18 044 A1), or to provide setting screws by means of which the spacing of the scale parts in the region of a joint can be adjusted (DE 27 27 769 C2). The additional outlay increases the production costs of the coordinate measuring machine in all cases.